1. Field of the Invention
The present invention relates to a membrane electrode assembly used for a fuel cell.
2. Description of Related Art
The increase in the amount of information caused by the recent progress of electronic technologies has yielded needs to process the increased information faster and more sophisticatedly. In order to respond to the needs, a power source of a high output density and a high energy density, namely a power source capable of continuous operation for a long time, is required.
There is an increasing demand for a small generator not requiring charge, namely a micro generator capable of being supplied with a fuel easily. Under the situation, the importance of a fuel cell has been worked on.
A fuel cell is a generator that comprises at least a solid or liquid electrolyte and two electrodes, an anode and a cathode, which cause intended electrochemical reactions; and directly converts the chemical energy of a fuel into electric energy with high efficiency.
Among such fuel cells, a fuel cell which uses a solid polymer electrolyte membrane as the electrolyte membrane and hydrogen as a fuel is called a solid polymer type fuel cell (PEFC: Polymer Electrolyte Fuel Cell). Then a fuel cell which uses methanol as a fuel is called a direct methanol type fuel cell (DMFC: Direct Methanol Fuel Cell). In particular, a DMFC that uses a liquid fuel attracts attention as being effective as a transportable or mobile small power source because the fuel has a high energy density per unit volume.
In a DMFC, a methanol crossover phenomenon wherein methanol supplied to the anode permeates the solid polymer electrolyte and reaches the cathode occurs. The methanol having reached the cathode is oxidized by oxygen supplied to the cathode and discharged as carbon dioxide. In the oxidation reaction process, intermediate products such as formic acid and formaldehyde are produced more than a little and discharged from the fuel cell. Further, there is some methanol that is discharged as it is from the fuel cell without being oxidized. Since methanol, formic acid and formaldehyde discharged from the fuel cell as stated above are harmful to human bodies, the quantities should be reduced to the utmost extent.
As an example of a method for removing methanol, formic acid and formaldehyde that are hazardous materials emitted from the cathode, there is a method of installing a filter containing a byproduct gas absorbent in an exhaust gas pipe of the cathode as described in (JP-A No. 2008-210796) Patent Document 1. Further, there is a method of installing a filter containing a catalyst to decompose the hazardous materials in the exhaust gas pipe as described in (JP-A No. 2005-183014) Patent Document 2.